Re: [syzbot] [bpf?] KCSAN: data-race in bpf_percpu_array_update / bpf_percpu_array_update (2)

[Daniel Borkmann]

On 8/29/23 8:53 PM, Marco Elver wrote:
> On Tue, 29 Aug 2023 at 20:30, Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
> > On 8/29/23 5:39 AM, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    727dbda16b83 Merge tag 'hardening-v6.6-rc1' of git://git.k..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=136f39dfa80000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=dea9c2ce3f646a25
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=97522333291430dd277f
> > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > Downloadable assets:
> > > disk image: https://storage.googleapis.com/syzbot-assets/9923a023ab11/disk-727dbda1.raw.xz
> > > vmlinux: https://storage.googleapis.com/syzbot-assets/650dbc695d77/vmlinux-727dbda1.xz
> > > kernel image: https://storage.googleapis.com/syzbot-assets/361da71276bf/bzImage-727dbda1.xz
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+97522333291430dd277f@xxxxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > ==================================================================
> > > BUG: KCSAN: data-race in bpf_percpu_array_update / bpf_percpu_array_update
> > >
> > > write to 0xffffe8fffe7425d8 of 8 bytes by task 8257 on cpu 1:
> > >    bpf_long_memcpy include/linux/bpf.h:428 [inline]
> > >    bpf_obj_memcpy include/linux/bpf.h:441 [inline]
> > >    copy_map_value_long include/linux/bpf.h:464 [inline]
> > >    bpf_percpu_array_update+0x3bb/0x500 kernel/bpf/arraymap.c:380
> > >    bpf_map_update_value+0x190/0x370 kernel/bpf/syscall.c:175
> > >    generic_map_update_batch+0x3ae/0x4f0 kernel/bpf/syscall.c:1749
> > >    bpf_map_do_batch+0x2df/0x3d0 kernel/bpf/syscall.c:4648
> > >    __sys_bpf+0x28a/0x780
> > >    __do_sys_bpf kernel/bpf/syscall.c:5241 [inline]
> > >    __se_sys_bpf kernel/bpf/syscall.c:5239 [inline]
> > >    __x64_sys_bpf+0x43/0x50 kernel/bpf/syscall.c:5239
> > >    do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > >    do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
> > >    entry_SYSCALL_64_after_hwframe+0x63/0xcd
> > >
> > > write to 0xffffe8fffe7425d8 of 8 bytes by task 8268 on cpu 0:
> > >    bpf_long_memcpy include/linux/bpf.h:428 [inline]
> > >    bpf_obj_memcpy include/linux/bpf.h:441 [inline]
> > >    copy_map_value_long include/linux/bpf.h:464 [inline]
> > >    bpf_percpu_array_update+0x3bb/0x500 kernel/bpf/arraymap.c:380
> > >    bpf_map_update_value+0x190/0x370 kernel/bpf/syscall.c:175
> > >    generic_map_update_batch+0x3ae/0x4f0 kernel/bpf/syscall.c:1749
> > >    bpf_map_do_batch+0x2df/0x3d0 kernel/bpf/syscall.c:4648
> > >    __sys_bpf+0x28a/0x780
> > >    __do_sys_bpf kernel/bpf/syscall.c:5241 [inline]
> > >    __se_sys_bpf kernel/bpf/syscall.c:5239 [inline]
> > >    __x64_sys_bpf+0x43/0x50 kernel/bpf/syscall.c:5239
> > >    do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > >    do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
> > >    entry_SYSCALL_64_after_hwframe+0x63/0xcd
> > >
> > > value changed: 0x0000000000000000 -> 0xfffffff000002788
> > >
> > > Reported by Kernel Concurrency Sanitizer on:
> > > CPU: 0 PID: 8268 Comm: syz-executor.4 Not tainted 6.5.0-syzkaller-00453-g727dbda16b83 #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/26/2023
> > > ==================================================================
> >
> > This case is with two tasks doing bpf_map batch update together for the
> > same map and key.
> >     > write to 0xffffe8fffe7425d8 of 8 bytes by task 8257 on cpu 1:
> >     > write to 0xffffe8fffe7425d8 of 8 bytes by task 8268 on cpu 0:
> >
> > So concurrency is introduced by user applications.
> > In my opinion, this probably not an issue from kernel perspective.
>
> Perhaps not, but I recall there being a discussion about making KCSAN
> aware of memory accesses done by BPF programs (memcpy being a tiny
> subset of those). Not sure if the above data race qualifies as
> something we might want to still detect, i.e. a kernel dev testing
> their kernel might be interested in such a report.
>
> Regardless, in this case we should teach syzkaller to ignore KCSAN
> data races that originate from bpf user operations whatever the
> origin.

I presume KCSAN could be silenced here via READ_ONCE/WRITE_ONCE conversion?

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index f58895830ada..32c4a37045f2 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -424,8 +424,11 @@ static inline void bpf_long_memcpy(void *dst, const void *src, u32 size)
 	long *ldst = dst;

 	size /= sizeof(long);
-	while (size--)
-		*ldst++ = *lsrc++;
+	while (size--) {
+		WRITE_ONCE(*ldst, READ_ONCE(*lsrc));
+		ldst++;
+		lsrc++;
+	}
 }

 /* copy everything but bpf_spin_lock, bpf_timer, and kptrs. There could be one of each. */